1. Field of the Invention
This invention relates generally to direct oxidation fuel cell systems, and more particularly to fuel cells in which an organic fuel in vaporous form is delivered to the anode of the fuel cell.
2. Background Information
Fuel cell power systems that convert an organic fuel such as methanol or ethanol and an oxidant into electricity are generally categorized into two types. In the first type, a fuel reformer is used to convert the organic fuel stream into a fuel stream containing hydrogen gas. The hydrogen gas is fed to the anode of a hydrogen-fueled fuel cell.
The second type is a direct oxidation fuel cell (DOFC) in which the organic fuel is reacted directly at an anode catalyst electrode of a membrane electrode assembly (MEA) of the fuel cell. An example of a direct oxidation fuel cell is the direct methanol fuel cell (DMFC). The half reactions for a DMFC are:Anode: CH3OH+H2O→CO2+6e−+6H+Cathode: 6e−+6H++ 3/2O2→3H2O
Many DMFC systems known in the art are liquid-feed systems that circulate a low-molarity methanol/water fuel solution through an anode flow field adjacent to an anode gas diffusion layer (GDL). Carbon dioxide (CO2) that is generated in the anode reaction exits through the anode flow field with the unused fuel solution where it is separated before the unused fuel solution is recirculated through the anode flow field.
Some liquid-feed DMFC systems operate using substantially 100% methanol and employ an active system to manage water in the fuel cell. Water is needed for the anode half reaction (as noted in the above reaction equations). Additionally, the cathode aspect of the membrane must be kept adequately hydrated, but not saturated or flooded. Thus, active water management systems are employed that include techniques for capturing water generated at the cathode and returning it to the anode. This replaces: (i) water lost to the anode reaction, (ii) water leaving the system through the CO2 vent, or (iii) water crossing over the polymer-electrolyte membrane (PEM) from the anode to the cathode. These active water management systems can become complex, adding costs, as well as size and weight, to a system that should be small and lightweight to satisfy commercial applications.
Furthermore, it has been found that DOFCs operate best when fuel and oxygen are delivered uniformly to an adequately-hydrated MEA. In a liquid-feed system, water is mixed with the fuel, which provides hydration of the PEM. In addition, fuel is provided in concentration levels adequate to evenly feed the full active area of the membrane. Concentration of the fuel can be managed so that the beginning of the flow path is not over concentrated and the end of the flow path is not under concentrated. In such cases, the energy required to distribute the fuel across the MEA active area comes from a liquid pump. But, these systems also require water delivery and/or recirculation mechanisms such as pumps and conduits for recirculating unused fuel and water back to the anode of the fuel cell.
It is also known to provide a direct-injection fuel feed DOFC in which liquid fuel is directly injected into the anode chamber of the fuel cell. In this case, any fuel that escapes unused is not captured and circulated back through the anode chamber. For example, U.S. Pat. No. 6,447,942 describes a direct methanol fuel cell in which liquid fuel is introduced to the anode by capillary action to a porous material that acts as a wick and which stays wetted with fuel. Another example of a direct-injection fuel cell system is commonly owned U.S. Pat. No. 6,981,877 of Ren et al., for a SIMPLIFIED DIRECT-OXIDATION FUEL CELL SYSTEM, which describes a direct-injection fuel feed system that feeds substantially 100% methanol to an anode chamber without active collection or pumping of water produced at the cathode. Other DOFCs provide fuel in evaporated methanol form to the anode. For example, commonly owned United States Published Patent Application No. US2005/0170224 of Ren et al., for a CONTROLLED DIRECT LIQUID INJECTION VAPOR FEED, describes a system in which liquid fuel is injected with a pump into an evaporator pad by a device with many injection ports; in another embodiment a dispersion member is placed between the evaporator pad and the anode GDL to effectively disperse the fuel.
Challenges are presented in such designs that include managing hydraulic and gravitational pressure in various orientations, as well as in providing components that adjust for the concentration of fuel in the evaporation pad being highest at the injection ports, in order to more uniformly distribute the fuel.
Notably, these prior techniques for direct injection of fuel feed in a vapor form each describe the liquid-to-vapor transition happening in close proximity to the anode GDL. In such designs, the fuel is distributed from a single point fuel source generally perpendicular to the active area of the fuel cell. However, because it is difficult to uniformly distribute the vaporous fuel, water can build up in areas where there is a lower concentration of fuel. Prior techniques attempt to mitigate the water problem by providing a dispersion member between an evaporation pad and the anode catalyst, however this still leaves void spaces in which water can collect. It has been found that the fuel diffuses through water droplets at a diffusion rate that is orders of magnitude lower than fuel diffusing through gas such as CO2. Thus, randomly distributed water droplets in the anode chamber void spaces can still result in a spatially non-uniform distribution of fuel to the anode catalyst which reduces performance.
In addition, there is also a temperature dependency that leads to degraded performance. More specifically, as noted, prior designs involve a liquid-to-vapor transition that happens in close proximity to the anode aspect of the MEA. The vapor delivery rate to the anode catalyst in such prior techniques is a function of the vapor pressure of fuel and the porosity of the fuel distribution layers. But the vapor pressure of the fuel is dependent upon the temperature at the area where the evaporation occurs. It has been found that, for a given porosity of layers between the liquid fuel and the anode catalyst, the vapor pressure of the vaporous fuel results in a desired fuel feed rate to the anode catalyst only at a single design point temperature. However, if the temperature in that area of the fuel cell is higher than this single design point temperature, then the vapor pressure is affected and a higher fuel-feed rate occurs. When the temperature is lower than the single design point temperature, then the vapor pressure is such that a lower fuel feed rate results. Thus, the vapor pressure and fuel feed rate are difficult to control due to this temperature dependency.
The temperature dependency can be worsened by the heat of the fuel cell operation itself. As the fuel cell reactions occur, heat can build up which may affect the temperature at the MEA, and cause the cathode to dry out.
Another problem is caused by the heat loss due to vaporization of the fuel acting to cool an area to a temperature that is lower then the membrane and catalyst layers. If the cooling is sufficient, then water generated by the fuel cell reaction at the MEA temperature may have a dew point that is higher than the temperature of the evaporation area of the fuel cell. This can result in condensation of water at the evaporator surface in the anode chamber, thus leading to the problems discussed above regarding build up of water in the active area of the anode.
Furthermore, an uneven distribution of fuel to the active area of the fuel cell, can lead to “hot spots,” which are locations on the membrane that have a much higher concentration of fuel than other places on the membrane. These “hot spots” result in uneven reactions at the catalyst face, degradation of the membrane due to high temperatures, and uneven generation of water which can shut down the electrochemical reaction at the localized area.
There remains a need, therefore, for a direct oxidation fuel cell system that has uniform fuel distribution from a single-point fuel injection. There remains a further need for a system in which a vaporous fuel is delivered at a desired vapor pressure in such a manner that the fuel feed rate that is controlled and does not depend upon the cell temperature. There remains yet a further need for a fuel cell system that includes heat and water management features that do not add complexity, weight, and/or size to the fuel cell system.